Acid loop libraries are basically in WAV format with a bit of "extra" data saved.... that is whether it is a one shot (like a drum hit) or a loop and if it is a loop, the key.
I was a bit disappointed in the Rock Orchestral... I like to buy stuff that I would have trouble recreating myself. So a loop of Middle Eastern Drums for example, where different types of hits at different velocities can really effect the sound, are superb. I thought I could do a lot of what was on the Rock Orchestral.... although using a real orchestra did make it sound... well, more realistic.
One other thought (whew, then I'm done thinking for the day)... check on the Sonic Foundry Website. You can download samples of each CD's sounds and also notice how much is on the CD. Some have as few as 121 loops, other have 3-400. Of course, if you really like the 120... it makes sense.
There are also a lot of loops for Acid that are not on the SF website. Plus, Acid (and it sounds like Sonar) is pretty darn good at reading plain old WAV files and figuring them out. The biggest problem with Acid is when it reads a non-looped wave and loops it.... like a high-hat being struck in really rapid succession. Otherwise, it is pretty cool.